That's not at all how backups work. In order for a hacker to download the and decrypt the account they would need to: 1. Access to your e-mail to click a link

That's not at all how backups work. In order for a hacker to download the and decrypt the account they would need to: 1. Access to your e-mail to click a link confirming you changed your cellphone. 2. Access to your phone # to be able to get the SMS message with the registration key. 3. The encryption key you used. If they can do all 3 they would get access to your account. If you feel that the level of security there is not good enough for you, you can disable the backups and key will never ever leave your phone